Brain protection by rapeseed oil in magnesium-deficient mice

Diets given for 30 days with various mono-(MUFA) and poly-(PUFA) unsaturated fatty acid contents were evaluated for brain protection in magnesium-deficient mice: a commercial and three synthetic diets (n-6PUFA, n-3PUFA and MUFA-based chows enriched with 5% corn/sunflower oils 1:3, with 5% rapeseed oil and with 5% high oleic acid sunflower oil/sunflower oil 7:3, respectively). Unlike magnesium deprivation, they induced significant differences in brain and erythrocyte membrane phospholipid fatty acid compositions. n-3PUFA but not other diets protected magnesium-deficient mice against hyperactivity and moderately towards maximal electroshock- and NMDA-induced seizures. This diet also inhibited audiogenic seizures by 50%, preventing animal deaths. Because, like n-6PUFA diet, matched control MUFA diet failed to induce brain protections, alpha-linolenate (ALA) rather than reduced n-6 PUFA diet content is concluded to cause n-3PUFA neuroprotection. Present in vivo data also corroborate literature in vitro inhibition of T type calcium channels by n-3 PUFA, adding basis to ALA supplementation in human anti-epileptic/neuroprotective strategies.     

Evidence for a Hyperexcitability State of Staggerer Mutant Mice Macrophages

We recently reported an abnormal production of interleukin-1 (IL-1) in peripheral macrophages of several neurological mutant mice that exhibit patterns of neuronal degeneration, especially in the cerebellum. After in vitro activation by lipopolysaccharide acid (LPS), these macrophages hyperexpress IL-1 beta mRNA and hyperproduce IL-1 protein in comparison with +/+ controls. In the present study, focused on the staggerer mutant mice, we investigate if this genetic dysregulation is specific for IL-1 beta or if it reflects a generalized hyperexcitability of these macrophages. The hyperexpression of IL-1 beta mRNA in sg/sg macrophages is present whatever the duration of LPS stimulation, even for periods as short as 15 min, although it reaches a maximum after 4 h of stimulation. The hyperinducibility of sg/sg macrophages is observed even when very low doses of LPS are used (0.01 microgram/ml) and reaches its maximum for 5 micrograms/ml LPS. Synthetic molecules (muramyl dipeptides), such as N-acetylmuramyl-L-alanyl-D-isoglutamine or murabutide, known as macrophage activators, are also efficient in revealing the cytokine hyperexpression in sg/sg macrophages. In addition, hyperexpression of two other cytokines, i.e., tumor necrosis factor-alpha and IL-1 alpha mRNAs, is also detected in LPS-stimulated macrophages of mutant mice. Finally, the effect of an inhibitor of protein synthesis, cycloheximide, is similar in +/+ and sg/sg macrophages. As a whole, these data lead us to conclude that the sg/sg macrophages are in a state of general hyperexcitability when compared with +/+ ones.     

Locomotor Dysfunction and Pain: The Scylla and Charybdis of Fiber Sprouting After Spinal Cord Injury

Injury to the spinal cord (SCI) can produce a constellation of problems including chronic pain, autonomic dysreflexia, and motor dysfunction. Neuroplasticity in the form of fiber sprouting or the lack thereof is an important phenomenon that can contribute to the deleterious effects of SCI. Aberrant sprouting of primary afferent fibers and synaptogenesis within incorrect dorsal horn laminae leads to the development and maintenance of chronic pain as well as autonomic dysreflexia. At the same time, interruption of connections between supraspinal motor control centers and spinal cord output cells, due to lack of successful regenerative sprouting of injured descending fiber tracts, contributes to motor deficits. Similarities in the molecular control of axonal growth of motor and sensory fibers have made the development of cogent therapies difficult. In this study, we discuss recent findings related to the degradation of inhibitory barriers and promotion of sprouting of motor fibers as a strategy for the restoration of motor function and note that this may induce primary afferent fiber sprouting that can contribute to chronic pain. We highlight the importance of careful attentiveness to off-target molecular- and circuit-level modulation of nociceptive processing while moving forward with the development of therapies that will restore motor function after SCI.     

Trichinella spiralis: skeletal muscle membrane potentials in infected and uninfected mice

Male albino mice were infected orally with 400 ± 10 excysted Trichinella spiralis larvae. Skeletal muscle resting membrane potentials were recorded from the tibialis anterior muscles of infected and uninfected mice on the following days postinfection (PI): 1–15, 18, 20, 24, 28, 30–60 (at 5-day intervals), 90, 120, 150, and 180. The membrane potentials were significantly (P < 0.05) lower in infected muscle (82 vs 85 mV) on Day 30 PI. On Days 60, 90, 120, 150, and 180 PI the mean membrane potential in infected muscle (62 mV) was about 23 mV lower than the mean for uninfected muscle (85 mV) and this difference was highly significant (P < 0.001). These findings are discussed in relationship to other physiological alterations known to occur in skeletal muscles infected with T. spiralis larvae.     

Inherited Pain: SODIUM CHANNEL NAV1.7 A1632T MUTATION CAUSES ERYTHROMELALGIA DUE TO A SHIFT OF FAST INACTIVATION*

Inherited erythromelalgia (IEM) causes debilitating episodic neuropathic pain characterized by burning in the extremities. Inherited “paroxysmal extreme pain disorder” (PEPD) differs in its clinical picture and affects proximal body areas like the rectal, ocular, or jaw regions. Both pain syndromes have been linked to mutations in the voltage-gated sodium channel Nav1.7. Electrophysiological characterization shows that IEM-causing mutations generally enhance activation, whereas mutations leading to PEPD alter fast inactivation. Previously, an A1632E mutation of a patient with overlapping symptoms of IEM and PEPD was reported (Estacion, M., Dib-Hajj, S. D., Benke, P. J., Te Morsche, R. H., Eastman, E. M., Macala, L. J., Drenth, J. P., and Waxman, S. G. (2008) NaV1.7 Gain-of-function mutations as a continuum. A1632E displays physiological changes associated with erythromelalgia and paroxysmal extreme pain disorder mutations and produces symptoms of both disorders. J. Neurosci. 28, 11079–11088), displaying a shift of both activation and fast inactivation. Here, we characterize a new mutation of Nav1.7, A1632T, found in a patient suffering from IEM. Although transfection of A1632T in sensory neurons resulted in hyperexcitability and spontaneous firing of dorsal root ganglia (DRG) neurons, whole-cell patch clamp of transfected HEK cells revealed that Nav1.7 activation was unaltered by the A1632T mutation but that steady-state fast inactivation was shifted to more depolarized potentials. This is a characteristic normally attributed to PEPD-causing mutations. In contrast to the IEM/PEPD crossover mutation A1632E, A1632T failed to slow current decay (i.e. open-state inactivation) and did not increase resurgent currents, which have been suggested to contribute to high-frequency firing in physiological and pathological conditions. Reduced fast inactivation without increased resurgent currents induces symptoms of IEM, not PEPD, in the new Nav1.7 mutation, A1632T. Therefore, persistent and resurgent currents are likely to determine whether a mutation in Nav1.7 leads to IEM or PEPD.     

Low Tryptophan and Protein in the Diet During Development Increase the Susceptibility to Convulsions in Adult Rats

Tryptophan (TRY) is the precursor for serotonin (5-HT) synthesis. Common maize has low protein content with low concentration of TRY and lysine. A diet based on two strains of corn differing in their TRY content were given to adult female rats, prior mating, during pregnancy and lactation. Same diets were offered to their male offspring after weaning until reaching 60-days old. The pattern and severity of the convulsive phenomenon induced by monosodium glutamate (MSG) in a well established model of Status epilepticus were evaluated in comparison with data from animals of two control groups: (a) rats fed a hypoproteic (8% protein) diet, and (b) rats fed a normal Purina chow diet (23% protein). Significant increased susceptibility to convulsions was observed in both groups of rats fed the corn-based diets. However, the animals fed the common corn-based diet (8–9% protein; 0.058% TRY) showed a higher susceptibility to convulsions than what was registered in animals fed a Quality Protein Maize (QPM)-based diet (8–9% protein; 0.1% TRY). It is concluded that low TRY concentration in the diet during development, produces lower rate of brain 5-HT synthesis, affecting development and maturation of GABAergic inhibitory cortical interneurons, with alteration of cortical excitability, contributing in part, to the increased susceptibility to convulsions, as shown in the experiments here reported. Special issue article in honor of Dr. Ricardo Tapia.     

Differential contributions of somatic and dendritic calcium-dependent potassium currents to the control of motoneuron excitability following spinal cord injury

The hyperexcitability of alpha-motoneurons and accompanying spasticity following spinal cord injury (SCI) have been attributed to enhanced persistent inward currents (PICs), including L-type calcium and persistent sodium currents. Factors controlling PICs may offer new therapies for managing spasticity. Such factors include calcium-activated potassium (KCa) currents, comprising in motoneurons an after-hyperpolarization-producing current (I _KCaN) activated by N/P-type calcium currents, and a second current (I _KCaL) activated by L-type calcium currents (Li and Bennett in J neurophysiol 97:767–783, 2007). We hypothesize that these two currents offer differential control of PICs and motoneuron excitability based on their probable somatic and dendritic locations, respectively. We reproduced SCI-induced PIC enhancement in a two-compartment motoneuron model that resulted in persistent dendritic plateau potentials. Removing dendritic I _KCaL eliminated primary frequency range discharge and produced an abrupt transition into tertiary range firing without significant changes in the overall frequency gain. However, I _KCaN removal mainly increased the gain. Steady-state analyses of dendritic membrane potential showed that I _KCaL limits plateau potential magnitude and strongly modulates the somatic injected current thresholds for plateau onset and offset. In contrast, I _KCaN had no effect on the plateau magnitude and thresholds. These results suggest that impaired function of I _KCaL may be an important intrinsic mechanism underlying PIC-induced motoneuron hyperexcitability following SCI.     

Gain-of-function mutation of Nav1.5 in atrial fibrillation enhances cellular excitability and lowers the threshold for action potential firing

Genetic mutations of the cardiac sodium channel (SCN5A) specific only to the phenotype of atrial fibrillation have recently been described. However, data on the biophysical properties of SCN5A variants associated with atrial fibrillation are scarce. In a mother and son with lone atrial fibrillation, we identified a novel SCN5A coding variant, K1493R, which altered a highly conserved residue in the DIII-IV linker and was located six amino acids downstream from the fast inactivation motif of sodium channels. Biophysical studies of K1493R in tsA201 cells demonstrated a significant positive shift in voltage-dependence of inactivation and a large ramp current near resting membrane potential, indicating a gain-of-function. Enhanced cellular excitability was observed in transfected HL-1 atrial cardiomyocytes, including spontaneous action potential depolarizations and a lower threshold for action potential firing. These novel biophysical observations provide molecular evidence linking cellular “hyperexcitability” as a mechanism inducing vulnerability to this common arrhythmia.     

Glutamate and tumor-associated epilepsy: Glial cell dysfunction in the peritumoral environment

Seizures are a serious and debilitating co-morbidity of primary brain tumors that affect most patients, yet their etiology is poorly understood. In many CNS pathologies, including epilepsy and brain injury, high levels of extracellular glutamate have been implicated in seizure generation. It has been shown that gliomas release neurotoxic levels of glutamate through their high expression of system xc-. More recently it was shown that the surrounding peritumoral cortex is spontaneously hyperexcitable. In this review, we discuss how gliomas induce changes in the surrounding environment that may further contribute to elevated extracellular glutamate and tumor-associated seizures. Peritumoral astrocytes become reactive and lose their ability to remove glutamate, while microglia, in response to signals from glioma cells, may release glutamate. In addition, gliomas increase blood brain barrier permeability, allowing seizure-inducing serum components, including glutamate, into the peritumoral region. These factors, working together or alone, may influence the frequency and severity of tumor-associated epilepsy.